1. Field of the Invention
The subject invention pertains to aqueous primers for metal parts to which composite or metal adherends will be bonded utilizing structural adhesives. More particularly, the subject invention pertains to primers comprising aqueous dispersions of solid epoxy resins and solid curing agents which contain little or no volatile organic solvent.
2. Description of the Related Art
In the current manufacture of composite structures, particularly in the aerospace and transportation sectors, it is frequently necessary to bond a fabricated metallic structure to metal or composite adherends utilizing structural adhesives, generally structural film adhesives, or to laminate one or more plies of thermosetting resin impregnated fibrous reinforcement to the fabricated metal structure. In general, to ensure the greatest level of adhesive strength, the metal surface(s) are scrupulously cleaned of dirt, soil, grease, and metal oxidation products immediately prior to bonding. Unfortunately, this procedure cannot be generally used in a great portion of manufacturing, as the cleaning and bonding operations are often separated significantly in time. During this time period, the metal surface may become reoxidized, lessening the adhesive strength of the bond.
To overcome this difficulty, it has been proposed to prime the cleaned surface. Unfortunately, the primer may not permit the same degree of adhesion to either the metal surface(s) or composite adherends, and thus the resulting joint may not be as strong as that which could be achieved on a freshly prepared surface, although still superior to that which could be obtained from an oxidized surface. The primer must also be receptive to paint as well as adhesives.
Primers have often been prepared from one or two component thermosetting resins diluted to sprayable levels with organic solvents. Liquid epoxy esters were proposed for use in coatings by C. K. Thorstad, "Emulsions--Why and How They are Used", Modern Plastics, July 1959, pp. 83-84, in compositions containing either water or the epoxy ester itself as a vehicle, together with polyvinylacetate, polyacrylic, or poly(butadiene/styrene) lattices. Acid curing agents, for example dimethyl acid pyrophosphate or boron trifluoride are cited for these applications. However, the use of primers containing large amounts of volatile organic compounds (VOCs) is under increasing scrutiny for both toxicological and environmental reasons.
It has been proposed to utilize electrodepositable primers. Such primers contain resin particles bearing a charged group which migrate to an oppositely charged substrate from aqueous solution or dispersion. Such compositions often contain a lower but still undesirable level of VOCs and do not have ideal properties due to the restraints placed on the resin structure. Furthermore, the metal parts must be completely immersed in a tank of primer which impractical for large parts such as aircraft wings or fuselages. To be economical, such baths must be continually monitored and refreshed or must be reprocessed.
In U.S. Pat. No. 2,872,427 it has been proposed to utilize aqueous emulsions of liquid epoxy resins in textile treating. Additional suggested uses are in paints and enamels. The emulsions contain one or more emulsified liquid epoxy resins dispersed in an aqueous dispersing phase containing an alkali and acid stable non-ionic emulsifying agent and a water-dispersible binding colloid. For those applications where curing agents are also required, the curing agent is dissolved into the epoxy resin prior to dispersion. Such emulsions are storage stable with respect to sedimentation or coagulation, but suffer from the defect that since the epoxy and curing agent are intimately associated, premature curing may take place, particularly when exposed to elevated temperatures during shipment or storage. Shelf life of such emulsions is extremely limited. Furthermore, the protective colloids utilized are not themselves thermally stable, and may cause high temperature performance of the composition to suffer. Finally, such coatings suffer from lack of solvent resistance.
Raghavon and Lewis in the article "Epoxy Water-Borne Primer: Low-Temp Cure and Zero VOCs", Modern Paint and Coatings, July 1991, evidences the need for water-borne primers without VOCs and suggest use of an aqueous dispersion. However, no details of the composition suitable for such use are given.
U.S. Pat. No. 5,001,173 describes basecoats for metal surfaces which are to be latex primed and painted. The basecoat comprises an aqueous epoxy resin dispersion also containing chromium trioxide and preferably phosphoric acid. The use of aromatic amine or other solid epoxy curing agents is not disclosed. The basecoat is followed by a solvent borne primer containing substantial amounts of polyesters.
In the article "Guidelines to Formulation of Waterborne Epoxy Primers", M. A. Jackson, Polymer Paint Colour Journal 180 (4270) (1990) at pages 608-621 are described two component primer systems containing as one component an epoxy resin dispersion in water and solvent together with various corrosion inhibitors, and as the second component, a water reducible amine catalyst in water. Such formulations require mixing or metering of two separate components and are not storage stable.
In the article "Waterborne Epoxy Dispersions Provide Compliant Alternatives", R. Buehner et. al., Adhesives Age, December 1991, are described waterborne liquid and solid epoxy resin dispersions cured with dicyandiamide and water soluble 2-methylimidazole catalyst for use as adhesives. However, the shelf life of such dispersions is indicated as only greater than several days.
It would be an advance in the art to prepare a one-component aqueous adhesive primer which contains little or no VOCs; which is storage stable with respect to premature curing for extended periods of time when suitably stored (for example, for a period in excess of 24 weeks when stored at temperatures below 5.degree. C., or for a period in excess of 3 months at ambient temperature); which allows for excellent adhesion of heat-curable resins to the metal substrate, preferably similar to that of a freshly cleaned metal surface; which can be easily and economically be prepared; and whose application to the substrate is practical and economical. These and other objects have been achieved by the use of an aqueous, non-ionic solid epoxy resin dispersion which contain as a distinct phase a solid epoxy curing agent, preferably in the substantial absence of any protective colloid.